Typically non reversal photographic black and white film or paper photographic processors comprise four distinct sections:
developer; PA1 fixer; PA1 wash; and PA1 dryer.
The film or paper being processed first passes into the developer section where the latent image formed by light exposure is converted chemically to metallic silver. The film exits the developer and passes into the fixer section where the silver halide crystals that were not converted to metallic silver are dissolved out of the product, usually by a sodium or ammonium thiosulfate solution. The product then exits the fixer bath into a wash water bath where excess fixer is removed from the film or paper.
The amount of wash water required varies extensively among photographic processors. In the graphic arts segment, water requirements vary between 3.8 to 9.5 liters per minute (1.0 and 2.5 gallons). Until about ten years ago, a typical processor could use 1,500 to 4,500 liters of water per eight hours (400 to 1,200 gallons).
As water scarcity and cost increased, photographic processors installed water-saver solenoids to prevent fresh water from being used except when film or paper was actually being processed. These solenoids significantly reduced the amount of water consumed but it still is common for a processor to use as much as 950 to 1900 liters (250-500 gallons) of water per 8 hour per day.
Silver thiosulfate complex is carried out of the fixer bath in to the wash water by photographic films and papers during processing. Typical silver concentrations in single-use wash waters range from 3 to &gt;10 mg/L (ppm). The used wash water is typically discharged to public or private sewers. Sewer codes have become increasingly strict over the past decade. It is not unusual to find sewer restrictions for silver between 1 and 5 mg/L in the U.S., Canada, and Western Europe. Land use restrictions for septic systems are even lower. Photoprocessors are slowly being restricted from discharging their used wash waters without prior treatment to remove silver. If the water must be hauled away from the photoprocessor for disposal, costs of $3 to $5 per gallon are typical.
Removal of silver thiosulfate ions using anion exchange resins from dilute aqueous solutions weakly basic is known in the art. However, as the concentration of thiosulfate increases, it impairs the effectiveness of such resins in removing silver thiosulfate ions from photographic wash water. At elevated concentrations the thiosulfate ions in solution displace silver thiosulfate ions from the resin. Other anions, such as halides, can have a similar effect.